backend/nodes cleanup: Move loop variables definitions into for statement

While working on the other patch [1] that touched tbm_add_tuples() under src/backend/nodes, I moved a loop variable into for statement by the way as I knew Peter Eisentraut had done some efforts about that, see [2].

However, Peter removed that change from the patch and he said that should belong to a separate cleanup patch. So I am following up and moving loop variables into for statements wherever possible under src/backend/nodes.

[1] https://www.postgresql.org/message-id/flat/CAEoWx2m2E0xE8Kvbkv31ULh_E+5zph-WA_bEdv3UR9CLhw+3vg@mail.gmail.com

[2] https://git.postgresql.org/cgit/postgresql.git/commit/?id=03fbb0814c5015ab79e670ab97bb6a3349269e4b

Best regards,